1. Technical Field
The present invention relates to an electrostatic transducer, and more particularly, to an electrostatic transducer for reproducing sound having sharp directivity by outputting a modulated wave obtained by modulating a carrier wave in an ultrasonic band with an acoustic signal in an audio frequency band, an ultrasonic speaker using the electrostatic transducer, a display device having the ultrasonic speaker, a directional acoustic system, and a driving circuit of a capacitive load.
2. Related Art
An ultrasonic speaker can reproduce sound having sharp directivity by outputting a modulated wave obtained by modulating a carrier wave in an ultrasonic band with an acoustic signal in audio frequency band. In a transducer (transmitter) of the ultrasonic speaker, a piezoelectric transducer is generally used. However, since the piezoelectric transducer uses a sharp resonance characteristic, a sound pressure is high, but a frequency band is narrow. Accordingly, in the ultrasonic speaker using the piezoelectric transducer, a reproducible frequency band is narrow and thus reproduction quality deteriorates compared with a loudspeaker.
Accordingly, there is provided an ultrasonic speaker (see an example of an electrostatic transducer according to the invention shown in FIGS. 1A to 1C) using an electrostatic transducer for vibrating a vibration membrane by applying an electrostatic force between a vibration membrane electrode and a fixed electrode to generate a sound pressure. The electrostatic transducer is characterized in that a flat output sound pressure characteristic can be obtained over a wide frequency range. Therefore, the ultrasonic speaker using the electrostatic transducer can provide superior reproduction quality compared with the piezoelectric transducer.
In the electrostatic transducer, a high voltage of at least several hundreds V need be applied between the vibration membrane electrode and the fixed electrode, in order to generate a high sound pressure. Accordingly, in order to drive the electrostatic transducer, a voltage is generally boosted by an output transformer (for example, see JP-A-6-209499).
Since the electrostatic transducer has the same configuration as that of a capacitor, a capacitance component dominates in an electrical characteristic of the transducer. Accordingly, when the electrostatic transducer is driven by the output transformer, a resonance circuit system is formed by an inductance component of the transformer and a capacitance component of the transducer. By the influence of the resonance, a frequency characteristic of a terminal voltage (output voltage) of the transducer significantly varies and thus a flat output characteristic cannot be obtained. Accordingly, the reproduction quality of the ultrasonic speaker deteriorates.
When a resonance frequency band can be significantly shifted from a driving frequency band, the frequency characteristic is made nearly flat. In the ultrasonic speaker, a driving frequency band is in an ultrasonic band. Accordingly, in order to significantly shift the resonance frequency band of a circuit from the driving frequency band to a high frequency band, a coil inductance of the output transformer must be very small and this is not realistic.
The coil inductance value of the output transformer and the capacitance value of the electrostatic transducer are restricted within realistic ranges due to a structural limit. Accordingly, by a combination the inductance value and the capacitance value, the resonance frequency band of the circuit is relatively in the vicinity of the driving frequency band. That is, when the output transformer is interposed, the output frequency characteristic of the ultrasonic speaker significantly varies.
By connecting a register to a primary side or a secondary side of the output transformer, the frequency characteristic of the output voltage can be made flat, but loss occurs by the resistor. This is not preferable because low power loss which is a feature of the electrostatic transducer is eliminated.
As the ultrasonic transducer in the related art, there is disclosed a configuration which is capable of matching impedance and ensuring the flat output frequency characteristic by implementing a butterworth filter using a circuit constant (an inductance component and a capacitance component) of a piezoelectric element which is a load (see JP-A-2001-86587). However, since the configuration of the ultrasonic transducer in the related art is premised on the driving of the piezoelectric element, a problem may occur when the configuration applies to the driving of the electrostatic transducer.
In order to obtain the flat pass band characteristic by the configuration of a T-type or π-type LC filter disclosed in JP-A-2001-86587, a relatively large resistance component is required. Since the piezoelectric element has the relatively large resistance component as an electrical characteristic of the load, the flat pass band characteristic is realized by using the (relatively large) resistance component of the piezoelectric element, which is the load, as a portion of the filter, in JP-A-2001-86587.
Meanwhile, the electrostatic transducer basically has the same configuration as that of a film capacitor. Accordingly, the capacitance component dominates in the electrical characteristic of the transducer and a resistance component is very small (compared with the piezoelectric transducer). Accordingly, when the circuit configuration disclosed in JP-A-2001-86587 applies to the electrostatic transducer, an external resistor having a relatively high resistance value must be added in order to obtain the flat pass band characteristic and thus unnecessary power loss occurs by the resistor. However, when the resistor is not added, the LC filter has a steep response characteristic (resonance curve) and thus the flat pass band characteristic cannot be obtained.